Biochemical liver function tests have been used by physicians in the identification and management of liver disease. These static tests include measurements of albumin, bilirubin, enzymes and prothrombin time. Although such measures provide information about the immediate functional state of the liver, they are rarely useful in the prediction for outcome of liver disease. Therefore, there is a need to develop a quantitative liver function test which would be more useful than standard liver tests to assess the degree and progression of hepatic dysfunction. We intend to develop a quantitative liver function test using a stable isotope breath test which will test functional capacity and integrity of the liver. This test will use a 13C-amino acid as the liver substrate and monitor oxidation to breath CO2. The test is based on experiments which show the usefulness of measuring amino acid clearance in cirrhotics. It is our hypothesis that measurement of amino acid oxidation is a measure of hepatocyte function and will decrease during liver dysfunction. The long term goal is to provide a test which has the ability to assess the severity of liver disease. This information will lead to appropriate treatment taken at an earlier point before complete decompensation occurs.